The present invention relates to a radio blind zone system having a master unit situated at an existing zone, a slave unit located at a radio signal blind zone, and a coaxial transmission path connecting the two units, the master unit feeding a load current to the slave unit. More particularly, the present invention is concerned with a device which allows the master unit to detect an error occurred in the slave unit on the basis of a change in the load current.
A conventional radio blind tone system of the kind described has a master unit, a slave unit including a relay amplifying device, a coaxial transmission path connecting the master and slave units, and an antenna. The master unit includes a radio signal input/output terminal, and a power source which feeds a DC current for driving the slave unit. A slave error detector monitors the load current of the slave unit and outputs, when it exceeds a predetermined threshold, an alarm signal. A mixer receives a radio signal and source current, superposes them on each other, and produces the resulting composite signal. The slave unit includes a separator to which the composite signal from the mixer is sent over the coaxial path The separator separates the original radio signal and source current included in the composite signal. Three transistors constitute a tristage amplifier circuit and receive the separated source current and the radio signal as a source current and an input, respectively. An exclusive terminal is connected to the antenna. The system having such a configuration is selectively applicable to radio relay transmission or a radio relay reception, as desired.
The three transistors of the slave unit have operation currents, under normal conditions, which are different from each other. Errors particular to the transistors include unusual oscillation and breakdown. Unusual oscillation causes the load currents of the transistors to increase while breakdown causes them to decrease. The master unit, feeding the source current to the slave unit, can constantly detect such changes in the load currents of the transistors. More specifically, the error detector of the master unit is responsive to changes in the load currents.
However, the conventional error detecting device described above has some problems yet to be solved, as follows. To begin with, when the first transistor, for example, is faulty, the resulting change in the small load current is too small, compared to the load current of the entire slave unit, to be detected as a change in load. Further, it is likely that an unusual change in the current of any one of the transistors is cancelled by a change in the current of another transistor attributable to a temperature change or is susceptible to noise. Under this condition, it is extremely difficult to detect an error occurred in the slave unit accurately.